President-elect Donald Trump had a potentially provocative phone conversation Friday with Tsai Ing-wen, the president of Taiwan, which could upset delicate relations between the U.S. and the Chinese government.

It is believed to be the first call between a president or president-elect with a Taiwanese leader since 1979, when the U.S. recognized the mainland government and cut ties with Taiwan.

China has long been sensitive to any diplomatic recognition of Taiwan, which it regards as a rogue province. The reaction from China could be sharp.

Trump repeatedly criticized China during his campaign, promising to brand the country a currency manipulator, which could also add tension to relations with the Pacific power. He blamed China for undermining U.S. manufacturing jobs by selling artificially cheap exports.

The Trump transition team confirmed the call Friday in a statement summarizing contacts with four foreign leaders, including leaders from Afghanistan, the Philippines and Singapore.

“President-elect Trump spoke with President Tsai Ing-wen of Taiwan, who offered her congratulations,” the statement said. “During the discussion, they noted the close economic, political, and security ties exists between Taiwan and the United States. President-elect Trump also congratulated President Tsai on becoming president of Taiwan earlier this year.”

Tsai won election in a landslide in January after running on a platform that included taking a harder line with China than her predecessor, including a refusal to adhere the tacit deal that only one China exists, united on both sides of the Strait of Taiwan.

But her popularity has plummeted amid economic malaise; more than 100,000 people protested outside her office in September. She could face growing pressure to propose conditions for dialogue with China.

The White House was not informed of the call in advance, Emily Horne, a spokeswoman for the National Security Council, said in a statement.

Less than two weeks ago, Chinese President Xi Jinping sat across from President Obama at an economic summit in Peru and declared that the U.S.-China relationship was at a “hinge moment” following the election of Trump.

“I hope the two sides will work together to focus on cooperation, manage our differences, and make sure there is a smooth transition in the relationship and that it will continue to grow going forward,” he said at the start of a bilateral meeting with Obama, their ninth face-to-face encounter.

Xi spoke with Trump days after he was elected. The Chinese president told Trump that cooperation was “the only correct choice for China and the United States,” Chinese state media reported.

The White House has not been getting direct reports about Trump’s calls with foreign leaders. In some cases, they have gotten informal summaries from foreign counterparts.

On Thursday, a day after Pakistan’s government released candid details of a call between Trump and Prime Minister Nawaz Sharif, White House Press Secretary Josh Earnest diplomatically counseled the incoming president and his team about the value of following State Department guidance about sensitive foreign contacts.

“Every president, regardless of which party they’re in, benefits enormously from the expertise and service of thousands of patriotic Americans at the State Department,” he said. “… And I’m confident that as President-elect Trump takes office, those same State Department employees will stand ready to offer him advice as he conducts the business of the United States overseas. Hopefully he’ll take it.”

3:35 p.m.: This story was updated with a statement from the National Security Council.